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The EU's Catherine Ashton and Iran's Mohammad Javad Zarif will begin the talks with a working dinner

Talks between six world powers and Iran on the country's controversial nuclear programme are to resume in Vienna.

Negotiators are expected to begin trying to draft an agreement that will provide a comprehensive solution to the Iranian nuclear issue.

Although meetings have been held since February, correspondents say nothing of substance has yet been agreed.

Both sides hope to build on an interim deal that saw uranium enrichment curbed by Iran in return for sanctions relief.

The accord - which was signed in Geneva in November but only took effect in January - gives them until late July to agree a comprehensive solution, although that deadline can be extended by mutual consent.

The world powers want Iran to scale back its sensitive nuclear activities permanently to ensure that it cannot assemble a nuclear weapon.

But Iran says its nuclear work, which it insists is peaceful, will continue - and wants an end to the sanctions that have crippled its economy.

'Transparency'

The four days of talks between Iran and the P5+1 - the US, UK, France, China and Russia plus Germany - will begin with a working dinner at Vienna's Palais Coburg hotel for Iranian Foreign Minister Mohammad Javad Zarif and EU foreign policy chief Catherine Ashton.

Diplomats say there is political will on both sides to reach an agreement, but that it will be very difficult to overcome differences on three key issues:

Iran's uranium enrichment capacity

The heavy-water reactor at Arak

And the possible military dimensions of Iran's nuclear programme.

On Monday, Iranian representatives attended a meeting at the headquarters of the International Atomic Energy Agency (IAEA) in Vienna.

The global watchdog was said to have sought clarification about Tehran's efforts to develop exploding bridge wire (EBW) detonators, which could be used in nuclear weapons.

Iran has agreed to provide such information by Thursday, as part of a series of steps to allay the P5+1's concerns about its nuclear activities. Other steps include allowing IAEA inspectors to visit the Saghand uranium mine and the yellowcake production plant at Ardakan, which happened last week.

On Sunday, President Hassan Rouhani warned that Iran would not surrender its "right" to nuclear development at the Vienna talks.

"If the world seeks good relations with Iran, it should choose the way of surrendering to Iran's rights, respecting the Iranian nation and praising Iranian scientists," he told a group of medical and nuclear experts.

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Iran's President Hassan Rouhani has said his country will never build nuclear weapons

"The Iranian nation has never been after a weapon of mass destruction since it does not see it as legitimate," he added. "We do not have anything on the table to submit to others except transparency."

Iran's Supreme Leader Ayatollah Ali Khamenei - who has the final say on nuclear matters - expressed further misgivings about the talks.

"I have always been in favour of initiative and negotiation in foreign policy," he said, quoted by Irna, "but one must not tie our country's needs and issues like sanctions to the talks. Officials should tackle the sanctions issue in another way."

He also described as "stupid and idiotic" suggestions that Iran curb its ballistic missile development programme, amid reports that Western powers wanted to include it on the agenda of the discussions in Vienna.